Quicksand and warning sign at a gravel quarry.

Quicksand is a hydrocolloid gel consisting of fine granular matter (such as sand or silt), clay, and salt water. When unperturbed, it often appears to be solid; however, even a minor (less than 1%) change in the stress on the quicksand will cause a sudden decrease in its viscosity. After the initial perturbation - such as a person attempting to walk on it - the water and sand in the quicksand separate and dense regions of sand sediment form; it is because of the formation of these high volume fraction regions that the viscosity of the quicksand seems to suddenly increase. In order to move within the quicksand, a person or object must apply sufficient pressure on the compacted sand to re-introduce enough water to liquefy it. The forces required to do this are quite large: to remove a foot from quicksand at a speed of one centimeter per second would require the same amount of force as "that needed to lift a medium-sized car."

It was commonly believed that the behavior of quicksand was due solely to saturated or supersaturated suspensions of granules in water. Pressure from underground sources of water would separate and suspend the granular particles, reducing the friction between them. As of September 2005, it has been shown that it is the presence of salt that is largely responsible. The stability of the colloidal quicksand is compromised by the presence of salt; increasing the likelihood of sand flocculation and the formation of the high viscosity regions of sediment responsible for quicksand's "trapping" power.

Quicksand is not as dangerous as depicted in many movies. As quicksand is rarely more than a few feet deep, there is usually little danger of sinking below the surface. Furthermore, even when the quicksand is deep enough, deliberate effort is required to sink below the surface. Quicksand is typically denser than the human body, meaning that a body is much more buoyant in quicksand than in water. Thus, the body will float quite easily in quicksand.

This was demonstrated in an episode of the TV series MythBusters and in an article in Nature. The sand's higher density will gradually push a human body upward, eventually allowing one to paddle toward more solid footing. Typically, the greatest danger of getting stuck in quicksand comes from exposure, starvation, flash flooding, or tidal flooding.

Quicksand can be found inland (on riverbanks, near lakes, or in marshes) or near the coast. It can also form when an earthquake increases groundwater pressure, forcing the water to the surface and causing soil liquefaction.

One region notorious for its quicksands is Morecambe Bay, England. As the bay is very broad and shallow, a person trapped by the quicksand would be exposed to the danger of the returning tide, which can come in quite rapidly.

In fiction

People falling into (and, unrealistically, being submerged in) quicksand or a similar substance is a trope of adventure fiction, notably in movies. According to Slate, this gimmick had its heyday in the 1960s, when almost 3% of all films showed someone sinking in mud, sand, or clay. For instance, T.E. Lawrence's servant boy Daud dies in quicksand in a scene in the 1962 movie, Lawrence of Arabia. The proliferation of quicksand scenes in movies has given rise to an internet subculture scene dedicated to the topic. One of the earliest popular fictional references to quicksand occurs in Les Miserables, wherein Victor Hugo dedicates two chapters to the subject. In Robert Louis Stevenson's The Pavilion in the Links quicksand plays an important role.

The Tamil language movie Mella Thirandhathu Kadhavu uses quicksand as a major plot point in the flashback parts of the protagonist's story. Quicksand also features heavily in the Bengali novel Chorabali (Bengali for "quicksand"), in the Byomkesh Bakshi detective series by Sharadindu Bandyopadhyay.

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